Cyclonic separating apparatus are well known and have uses in a wide variety of applications. Over the last decade or so, the use of cyclonic separating apparatus to separate particles from an airflow in a vacuum cleaner has been developed and introduced to the market. Detailed descriptions of cyclonic separating apparatus for use in vacuum cleaners are given in, inter alia, U.S. Pat. Nos. 3,425,192 and 4,373,228 and EP 0 042 723. From these and other prior art documents, it can be seen that it is known to provide two cyclone units in series so that the airflow passes sequentially through at least two cyclones. This allows the larger dirt and debris to be extracted from the airflow in the first cyclone, leaving the second cyclone to operate under optimum conditions and so effectively to remove very fine particles in an efficient manner. This type of arrangement has been found to be effective when dealing with airflows in which is entrained a variety of matter having a wide particle size distribution. Such is the case in vacuum cleaners.
It is also desirable for vacuum cleaners to be both compact and energy efficient. A further desirable feature is a large capacity for collecting dirt and debris to reduce the frequency of emptying. In some known arrangements, the downstream cyclone has been placed inside the upstream cyclone in an attempt to minimize the size of the cleaner (see, for example, U.S. Pat. No. 4,373,228 and EP 0 042 723). However, this reduces the capacity of the cleaner because the downstream cyclone occupies a space which would otherwise be available for dirt and dust collection. In arrangements of the type shown in U.S. Pat. No. 3,425,192, the downstream cyclones are located outside the upstream cyclone but the partially cleaned air exiting from the upstream cyclone must then travel some distance to the inlets of the downstream cyclones. This increases the pressure drop across the system as a whole and thus reduces the energy efficiency of the system. Furthermore, the volume of the means for conducting the partially cleaned air adds to the overall volume of the machine.